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Stand outdoors to see and feel the radiation pouring from the Sun.
Most of the energy of the Universe is transported in this way, by radiation.
The violet limit therefore falls at 4000 A and the red limit near 7000 A or a bit longer.
Infrared runs from the red limit to about 0.1 millimeter, and the radio to as long as you wish, even to kilometers.
By the turn of the 20th century, spectra were being recorded photographically.
In the middle of the century, prisms were replaced by "diffraction gratings," finely ruled surfaces that produce spectra by the interference of light waves.
If with superhuman eyes you could see beyond red, you would encounter the "infrared" -- felt as heat on the skin -- which would merge gradually into the familiar "radio" portion of the spectrum.Violet light is slowed in a glass of water significantly more than red light.As a result shorter visual waves refract more than do longer ones."Spectra" is embedded with links that will take you back to the appropriate parts of the above two sites. The classic colors red, orange, yellow, green, blue, and violet connect in a seemingly infinite number of shades, one blending smoothly into the next.Together they constitute the "visual spectrum" (or "optical spectrum") because it is the part of the full spectrum that is seen with the human eye.
Ultraviolet runs from 4000 A down to about 100 A, X- rays take over to about 1 A, and these are followed by the gamma rays to no known lower limit.